When subjecting the velocity of a servomotor to feedback control, it is necessary to sense the actual velocity of the motor. To this end, conventional practice is to use a rotary encoder, by way of example. The rotary encoder is adapted to generate from several thousand to 20,000 pulses per revolution of the motor, the frequency of these output pulses being proportional to the rotational velocity of the motor.
Accordingly, the spacing between adjacent output pulses becomes very wide and the pulses become discrete in nature, especially when the servomotor rotates at a low velocity. When these pulses are used as motor rotation information in digital control, irregular rotation is the result.
Thus, when a rotary encoder for sensing the position of a mechanical load is used for sensing the velocity of a servomotor, velocity resolution is poor in comparison with an analog sensor and highly precise, smooth velocity control cannot be carried out.